1. Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
Silicon carbide (SiC) can have a high electric field breakdown strength and can be used as a material of a power device. Since an SiC semiconductor device has a high electric field breakdown strength, the SiC semiconductor device can control a high electric current. Thus, the SiC semiconductor device can be used for controlling a motor of a hybrid vehicle, for example.
In order to increase electric current that flows in a semiconductor device, increasing channel density is effective. In a silicon semiconductor device, a metal-oxide semiconductor field-effect transistor (MOSFET) having a trench gate structure is in practical use. A MOSFET having a trench gate structure is also applied, to an SiC semiconductor device. In the SiC semiconductor device, a channel forming plane of a trench gate structure of a MOSFET can be formed in a direction parallel to a {11-20} plane or a {1-100} plane.
U.S. Pat. No. 5,744,826 (corresponding to JP-A-9-199724) discloses a method of controlling a threshold value and an on-resistance of a MOSFET having a trench gate structure in an SiC semiconductor device. In the method, after forming trenches in an SiC substrate, an N type epitaxial layer made of SiC is formed on sidewalls of the trenches, that is, channel forming planes so as to form an accumulation channel. The threshold value and the on-resistance can be controlled by setting a thickness and an impurity concentration of the epitaxial layer optionally.
When the epitaxial layer is formed in the trenches by the conventional technique, it is difficult to form the epitaxial layer only on the sidewalls as the channel forming planes, and the epitaxial layer grows on a surface of the substrate, the sidewalls of the trenches, and bottom surfaces of the trenches along shapes of the trenches. In a MOSFET having a trench gate structure, the epitaxial layer that grows on the surface of the substrate is required to be removed so that an N+ type source region and a P+ type contact portion coupled with a P type base region, which are located under the epitaxial layer, can be electrically coupled with a source electrode. If the epitaxial layer that grows on the surface of the substrate is not removed completely and remains, a contact resistance increases.
The epitaxial layer formed on the sidewalls of the trenches are required to have predetermined thickness so that the threshold value of the trench gate structure of the MOSFET and a mobility can be controlled appropriately.
As described above, it is important to remove the epitaxial layer on the surface of the substrate appropriately and form the epitaxial layer on the sidewalls of the trenches so as to have the predetermined thickness.
However, a growth rate of the epitaxial layer varies among batches and in a plane. Therefore, it is difficult to control the thickness of the epitaxial layer appropriately unless the epitaxial layer is observed in a manufacturing process.